The present invention relates generally to power conversion apparatus and more particularly to a method and apparatus for precisely determining the instant in time at which each of several controlled switching devices of a controllable power conversion bridge is commutated (rendered non-conductive).
In the discipline of power conversion; e.g., converting alternating current (AC) to direct current (DC) or DC to AC, it is customary to employ a bridge arrangement of semiconductor devices (e.g., thyristors or transistors) to control the magnitude and/or frequency of electrical power delivered from a source to the load. A typical load in such an application is an electrical motor. In such conversion systems it is often necessary to determine the precise instant in time when the individual semiconductor devices are commutated (i.e., rendered non-conductive or turned off). Such a requirement is found, for example, in the invention which is the subject of U.S. patent application Ser. No. 458,108, "Method for Adaptive Conduction Control in a Power Converter" by W. D. Brackman, Jr., et al., filed Jan. 14, 1983, (assigned to the assignee of the present invention).
The problem of determining the time of commutation, particularly when the converter output current becomes discontinuous, is complicated by disturbances which are present in the converter output. These disturbances can arise from a large variety of causes including source disturbances, load disturbances and most especially those caused by the commutation of the devices of the bridge or by the commutation of switching devices of other bridges connected to the same source. While a number of commutation detection schemes are available, most tend to be inaccurate or very expensive. For example, current transformers can be used in the lines connecting the converter to the load; but, current transformers are magnetic devices and tend to be expensive as well as highly susceptible to electrical disturbances or electrical noise. It is also known in the art to detect the voltage across each of the several semiconductors (e.g., thyristors) of a bridge circuit and to determine the conductive state of each of these devices by that voltage. This, obviously, requires a large amount of detection circuitry and hence is relatively expensive.